Monazite RW-1: a homogenous natural reference material for SIMS U–Pb and Th–Pb isotopic analysis

Abstract

Well-characterized matrix-matched natural mineral references of known age are an important prerequisite for SIMS (secondary ion mass spectrometry) U–Th–Pb dating. We have characterized RW-1, a 44 g yellowish-brown single monazite specimen from a Norwegian pegmatite as an excellent hi-Th reference material for secondary ion mass spectrometric U–Th–Pb dating. A total of 206 secondary ion mass spectrometric analyses over six analytical sessions were performed on different monazite fragments of RW-1. The analyses resulted in 207Pb-based common lead corrected 206Pb/238U ages and Th–Pb ages with overall 2 % (2 SD = standard deviation) variations, indicating the good U–Th–Pb system homogeneity. The homogeneity of Th content of 11.8 ± 1.0 wt% (2 SD) and Th/U of 42 ± 3 (2 SD) make this crystal also a good compositional reference material. We used the combined ID–TIMS(Pb)/ID–MC–ICP–MS(U) technique (i.e. isotope dilution thermal ionization mass spectrometry for Pb, and isotope dilution multi-collector inductively-coupled plasma mass spectrometry for U) to determine U–Pb ages of the monazite samples studied. The mean 207Pb/235U age of 904.15 ± 0.26 Ma (95 % confidence level) is recommended as the best estimate crystallization age for RW-1 monazite. Considering that the most commonly distributed U–Pb monazite reference materials have rather low ThO2, we suggest that this RW-1 monazite with its ThO2 of 13.5 wt% is a suitable reference material providing investigators more confidence when dating high-Th monazite unknowns.